Method and apparatus for controlling engine exhaust sound for vehicles

ABSTRACT

An exhaust sound controlling method including reducing a flow rate of exhaust gas that passes through an exhaust path, to a first flow rate in accordance with increase in engine speed in a first engine speed region, and increasing the flow rate of exhaust gas to a second flow rate in accordance with increase in engine speed in a second engine speed region higher in engine speed than the first engine speed region. An exhaust sound controlling apparatus including a baffle plate and a valve that is operated to open a communication passage that communicates upstream and downstream sides of the baffle plate in an engine starting region, close the communication passage in an idling region and a low engine speed region, and open the communication passage and increase an opening of the valve as the engine speed rises in a high engine speed region.

BACKGROUND OF THE INVENTION

The present invention relates to method and apparatus for controllingengine exhaust sound that is emitted from an exhaust system of vehiclessuch as an automobile.

There have been conventionally proposed engine exhaust sound controllingapparatuses for vehicles which reduce exhaust sound in high engine speedregion as well as in low engine speed region while reducing exhaust lossin high engine speed region.

Japanese Patent Application First Publication No. 8-109815(corresponding to U.S. Pat. Nos. 5,614,699 and 5,739,483) indicatesengine exhaust sound controlling apparatus for vehicles which includes avalve for conducting changeover of exhaust paths in response to exhaustpressure, a biasing member such as a spring which biases the valve in aclosing direction of the valve against the exhaust pressure, and aspring constant changing member that reduces an increment of a reactionforce of the biasing member when a rotational angle of the valve exceedsa predetermined value.

SUMMARY OF THE INVENTION

Recently, it has been required to reduce back pressure and vary a levelof exhaust sound in an exhaust system in response to a vehicle runningstate. Specifically, when a vehicle engine is started, the exhaust soundwould be increased to create a powerful sound to be heard by a passengerof the vehicle. When the engine is operated in an idling region and alow engine speed region, i.e., a slow acceleration state, the exhaustsound would be reduced to ensure a silence within the interior of thevehicle. Further, when the engine is operated in a high engine speedregion, i.e., a rapid acceleration state, the exhaust sound would beincreased again to create a good acceleration sound to be heard by thepassenger.

However, in the apparatus of the conventional art as described above,when the engine is operated in the low engine speed region, the valve iskept in the closed state by the biasing member until a pressure in amuffler reaches a certain pressure value in order to ensure the silencewithin the interior of the vehicle. Further, in the conventionalapparatus, the valve is kept closed even in the engine starting region.Thus, the conventional art shows the exhaust system that is constructedfrom a viewpoint of reducing the exhaust sound. Therefore, the apparatusof the conventional art fails to produce a powerful exhaust sound in theengine starting region.

It is an object of the present invention to provide a method andapparatus for controlling exhaust sound which is capable of producing apowerful exhaust sound in an engine starting region and ensuring silencein the interior of the vehicle in a low engine speed region.

In one aspect of the present invention, there is provided a method forcontrolling exhaust sound from an engine which is emitted through anexhaust path, the method comprising:

a first control step of reducing a flow rate of exhaust gas that passesthrough the exhaust path, to a first flow rate in accordance withincrease in engine speed in a first engine speed region; and

a second control step of increasing the flow rate of exhaust gas to asecond flow rate in accordance with increase in engine speed in a secondengine speed region in which the engine speed is higher than that in thefirst engine speed region.

In a further aspect of the present invention, there is provided anapparatus for controlling exhaust sound from an engine which is emittedthrough an exhaust path, the apparatus comprising:

a baffle plate that is disposed within the exhaust path so as to block aflow of exhaust gas in the exhaust path, the baffle plate being formedwith an aperture;

a communication passage that communicates an upstream side of the baffleplate and a downstream side of the baffle plate with each other;

a valve that is so disposed as to cover a part of the aperture of thebaffle plate and operated to open and close the communication passage;and

wherein the valve is operated to open the communication passage in anengine starting region, the valve is operated to close the communicationpassage in an idling region and a low engine speed region, and the valveis operated to open the communication passage and increase an opening ofthe valve as the engine speed rises in a high engine speed region.

In a still further aspect of the present invention, there is provided anapparatus for controlling exhaust sound from an engine which is emittedthrough an exhaust path, the apparatus comprising:

a baffle plate that is disposed within the exhaust path so as to block aflow of exhaust gas in the exhaust path, the baffle plate being formedwith an aperture;

a valve that is so disposed as to cover a part of the aperture of thebaffle plate, the valve being rotationally moveable relative to thebaffle plate so as to be apart from the baffle plate in a direction ofthe flow of exhaust gas,

a biasing means for biasing the valve toward the baffle plate; and

an enclosure member that extends from the baffle plate so as to beopposed to the outer periphery of the valve when the valve is movedrelative to the baffle plate,

wherein the valve has a contact position in which the valve is contactedwith the baffle plate and covers the part of the aperture of the baffleplate, a small distance position in which the valve is apart from thebaffle plate in the direction of the flow of exhaust gas with a firstdistance therebetween, an intermediate distance position in which thevalve is apart from the baffle plate in the direction of the flow ofexhaust gas with a second distance therebetween which is larger than thefirst distance, and a large distance position in which the valve isapart from the baffle plate in the direction of the flow of exhaust gaswith a third distance therebetween which is larger than the firstdistance and the second distance, and

wherein when the valve is placed in the contact position, the valvecooperates with the baffle plate to define an opening between the outerperiphery of the valve and a periphery of the aperture of the baffleplate,

when the valve is placed in the small distance position, the valvecooperates with the enclosure member to define a first clearance betweenthe outer periphery of the valve and an opposing surface of theenclosure member that is opposed to the outer periphery of the valve,

when the valve is placed in the intermediate distance position, at leasta part of the outer periphery of the valve is substantially contactedwith the opposing surface of the enclosure member, and

when the valve is placed in the large distance position, the valvecooperates with the enclosure member to define a second clearancebetween the outer periphery of the valve and the opposing surface of theenclosure member, and the second clearance is increased as the thirddistance between the valve and the baffle plate becomes larger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a valve structure for use in an exhaust soundcontrol apparatus of an embodiment of the present invention.

FIG. 2 is a sectional view of the valve structure, taken along line 2-2of FIG. 1.

FIG. 3 is an explanatory diagram illustrating an operation of a valve ofthe valve structure of FIG. 1.

FIG. 4 is a sectional view of a muffler to which the exhaust soundcontrol apparatus of the embodiment is applied.

FIG. 5 is a diagram showing a relationship between engine speed andopening of the valve.

FIG. 6 is a diagram showing a relationship between a flow rate ofexhaust gas and a pressure forward of the valve.

FIG. 7 is a front view of a modification of the valve structure shown inFIG. 1.

FIG. 8 is a sectional view of the modified valve structure, taken alongline 8-8 of FIG. 7.

FIG. 9 is an explanatory diagram illustrating an operation of themodified valve structure of FIG. 7.

FIG. 10 is a sectional view of a modification of the muffler shown inFIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 to FIG. 3, an exhaust sound controlling apparatusfor vehicles, according to an embodiment of the present invention, nowis explained. The exhaust sound controlling apparatus includes a valvestructure shown in FIG. 1 and FIG. 2. As shown in FIG. 1 and FIG. 2, thevalve structure includes valve 2 that is disposed on baffle plate 1.Baffle plate 1 is disposed within a muffler as explained later which isconnected to a vehicle engine. Baffle plate 1 divides an interior of themuffler into a plurality of expansion chambers which constitute anexhaust path through which a flow of exhaust gas from the engine passes.Baffle plate 1 is so arranged as to block the flow of exhaust gas in theexhaust path.

Baffle plate 1 includes a generally rectangular-shaped aperture la asshown in FIG. 1. Aperture la extends through baffle plate 1. Valve 2 isdisposed on a downstream side surface of baffle plate 1 with respect tothe flow of exhaust gas and so arranged as to cover a part of aperture 1a. Specifically, a pair of supports 3 a, 3 b are arranged in a spacedand opposed relation to each other. Supports 3 a, 3 b are fixed tobaffle plate 1 by a suitable fastening manner such as welding. Valveshaft 4 is supported at opposite end portions thereof by supports 3 a, 3b. Valve 2 is rotatably mounted onto valve shaft 4.

Valve 2 is rotationally moveable relative to baffle plate 1 so as to beapart from baffle plate 1 in a direction of the flow of exhaust gas.Valve 2 is operated to open and close a communication passage whichallows fluid communication between an upstream side expansion chamberand a downstream side expansion chamber which are disposed on anupstream side and a downstream side of baffle plate 1 with respect tothe flow of exhaust gas in the exhaust path.

Specifically, valve 2 is a generally rectangular-shaped plate member.Valve 2 includes curved end portion 2 a at one end thereof, a pair ofsupport portions 2 b disposed on opposite sides of curved end portion 2a and planar valve body 2 c connected with curved end portion 2 a. Valve2 has end peripheral surface 2 d and two side peripheral surfaces whichare disposed on an outer periphery of valve body 2 c. End peripheralsurface 2 d is located at an opposite end of valve 2, namely, at a lowerend of valve 2 when viewed in FIG. 2. The side peripheral surfaces areopposed to each other in the axial direction of valve shaft 4 and joinedwith end peripheral surface 2 d. Curved end portion 2 a is raised upfrom valve body 2 c and formed into a curved shape. Support portions 2 bare spaced from each other in an axial direction of valve shaft 4 andlocated between supports 3 a, 3 b. Valve shaft 4 extends through shaftinsertion holes of support portions 2 b and is supported by supportportions 2 b. Valve body 2 c serves as a closing member that comes intocontact and non-contact with baffle plate 1 so as close the part ofaperture 1 a.

Curved end portion 2 a is so curved as to have a predetermined radiusthat extends from a central axis of valve shaft 4. An outercircumferential surface of curved end portion 2 a is opposed to baffleplate 1 and substantially in slide contact with baffle plate 1 duringthe rotational movement of valve 2 about valve shaft 4.

Torsion spring 5 is mounted to valve shaft 4 and biases valve body 2 cof valve 2 toward baffle plate 1. When valve 2 is biased by torsionspring 5 so as to be placed in a position shown in FIG. 2, valve body 2c is contacted near curved end portion 2 a with a part of baffle plate 1which is located along an upper periphery of aperture 1 a. Thus, aplaner upstream side surface of valve body 2 c is contacted with thedownstream side surface of baffle plate 1 exclusive of aperture 1 a. Inother words, when valve 2 is urged to be in the position shown in FIG.2, the upstream side surface of valve body 2 c is aligned with thedownstream side surface of baffle plate 1.

Aperture 1 a is so configured as to be partially covered by valve body 2c of valve 2. Specifically, aperture 1 a extends in the axial directionof valve shaft 4 so as to have an opening width equal to or slightlysmaller than a width of valve 2 as shown in FIG. 1. As shown in FIG. 2,aperture 1 a also extends in a direction perpendicular to the axialdirection of valve shaft 4 beyond end peripheral surface 2 d of valve 2.A part of aperture 1 a which is located on an outside of end peripheralsurface 2 d of valve 2, namely, on a lower side of end peripheralsurface 2 d in FIG. 2, serves as opening 10 through which an upstreamside expansion chamber and a downstream side expansion chamber disposedon an upstream side and a downstream side of baffle plate 1 with respectto the flow of exhaust gas in the exhaust path are communicated witheach other when valve 2 is urged to be in the position shown in FIG. 2.Specifically, when valve 2 is biased by torsion spring 5 and placed inthe position shown in FIG. 2, valve body 2 c is in contact with baffleplate 1 and aperture 1 a is covered by valve body 2 c except for opening10. Opening 10 is disposed between end peripheral surface 2 d and anaperture forming rim of baffle plate 1 which defines a periphery of therectangular-shaped aperture la. Opening 10 is always prevented frombeing covered by valve body 2 c.

Valve guides 6, 7 and 8 serve as an enclosure member that extends frombaffle plate 1 so as to surround the outer periphery of valve 2. Valveguides 6, 7 and 8 cooperate with baffle plate 1 and valve 2 to definethe communication passage that allows the fluid communication betweenthe upstream side expansion chamber disposed on the upstream side ofbaffle plate 1 and the downstream side expansion chamber disposed on thedownstream side of baffle plate 1.

Specifically, side valve guides 6 and 7 are disposed on both sides ofvalve 2 in an opposed and spaced relation thereto in the axial directionof valve shaft 4. Side valve guides 6 and 7 are formed into a plateshape and extend from the downstream side surface of baffle plate 1 inthe direction of the flow of exhaust gas, namely, in a rotationaldirection of valve 2, so as to be opposed to the side peripheralsurfaces of valve 2 during the rotational movement of valve 2 aboutvalve shaft 4. End valve guide 8 is interposed between side valve guides6 and 7 on the side of the other end of valve 2.

End valve guide 8 is opposed to and spaced from end peripheral surface 2d of valve 2 in the direction perpendicular to the axial direction ofvalve shaft 4. End valve guide 8 uprightly extends from the downstreamside surface of baffle plate 1 and has a predetermined height. End valveguide 8 extends along the bottom-linear part of the generallyrectangular-shaped periphery of aperture 1 a in the axial direction ofvalve shaft 4. End valve guide 8 has clearance forming surface 8 a andcontact surface 8 b on a side of valve 2 and valve shaft 4, namely, onan upper side of end valve guide 8 in FIG. 1 and FIG. 2. Specifically,clearance forming surface 8 a extends uprightly from the downstream sidesurface of baffle plate 1. Clearance forming surface 8 a cooperates withend peripheral surface 2 d of valve 2 to form the clearance therebetweenwhen valve 2 is rotated about valve shaft 4 and moved from the positionshown in FIG. 2 in an opening direction thereof in which valve body 2 cmoves apart from baffle plate 1 to uncover aperture 1 a. Contact surface8 b is connected with upright surface 8 a through a step portion betweenclearance forming surface 8 a and contact surface 8 b. Contact surface 8b is so curved as to be approximately close to or contacted with endperipheral surface 2 d of valve 2 during a further rotational movementof valve 2 about valve shaft 4 subsequent to passing through aboveclearance forming surface 8 a. Thus, end peripheral surface 2 d of valve2 is approximately close to or in contact with contact surface 8 b ofend valve guide 8 when valve 2 is further rotated about valve shaft 4subsequent to passing through above clearance forming surface 8 a of endvalve guide 8.

Valve 2 serves as a control valve that controls the flow of exhaust gaspassing through the exhaust path. An operation of valve 2 is explainedhereinafter with reference to FIG. 3. As shown in FIG. 3, valve 2 isurged to be placed in position A by the biasing force of torsion spring5. In position A, valve body 2 c is in contact with baffle plate 1, andaperture 1 a of baffle plate 1 is covered with valve body 2 c except foropening 10 that is disposed on the outside of end peripheral surface 2 dof valve 2. In position A, opening 10 is prevented from being coveredwith valve body 2 c and there is a clearance between end peripheralsurface 2 d of valve 2 and clearance forming surface 8 a of end valveguide 8. That is, in position A, valve 2 is in a first open state inwhich there is no distance between valve body 2 c and baffle plate 1 inthe direction of the flow of exhaust gas, namely, in the rotationaldirection of valve 2, and the expansion chambers on the upstream anddownstream sides of baffle plate 1 are communicated with each otherthrough opening 10 and the clearance between end peripheral surface 2 dand clearance forming surface 8 a.

Next, as shown in FIG. 3, when an external force, i.e., an exhaustpressure, is applied to valve 2 in the direction of the flow of exhaustgas in which valve 2 moves apart from baffle plate 1, valve 2 is rotatedabout valve shaft 4 against the biasing force of torsion spring 5 andmoved from position A to a position immediately before position B. Atthis time, a distance between baffle plate 1 and valve body 2 c of valve2 in the direction of the flow of exhaust gas is relatively small and anamount of the rotational movement of valve 2, namely, an opening ofvalve 2, is relatively small. During the rotational movement of valve 2,there exists the clearance between end peripheral surface 2 d of valve 2and clearance forming surface 8 a of end valve guide 8. The fluidcommunication between the upstream side expansion chamber and thedownstream side expansion chamber which are disposed on the upstreamside and the downstream side of baffle plate 1 can be maintained throughopening 10 and the clearance. Thus, valve 2 can be kept in the firstopen state while valve 2 is moved from position A to the positionimmediately before position B.

When the external force that is applied to valve 2 in the openingdirection is increased, valve 2 is further rotated about valve shaft 4against the biasing force of torsion spring 5. Valve 2 is placed inposition B and then moved from position B to position C. At this time,the rotation amount of valve 2 becomes intermediate and the distancebetween baffle plate 1 and valve body 2 c in the direction of the flowof exhaust gas becomes intermediate. During the rotational movement ofvalve 2 from position B to position C, at least a part of the outerperiphery of valve 2, namely, end peripheral surface 2 d of valve 2, isin substantially contact, namely, slide contact, with contact surface 8b of end valve guide 8. Therefore, the upstream side expansion chamberon the upstream side of baffle plate 1 can be prevented from beingcommunicated with the downstream side expansion chamber on thedownstream side of baffle plate 1. Thus, during the rotational movementof valve 2 from position B to position C, valve 2 is restrained from thefirst open state and kept in a closing state in which there is theintermediate distance between valve body 2 c and baffle plate 1 in thedirection of the flow of exhaust gas and the fluid communication betweenthe upstream side expansion chamber and the downstream side expansionchamber which are disposed on the upstream and downstream sides ofbaffle plate 1 can be substantially prevented.

When the external force is further increased, valve 2 is further rotatedabout valve shaft 4 against the biasing force of torsion spring 5 andmoved from position C in the opening direction to thereby be apart fromend valve guide 8. At this time, the rotation amount of valve 2 becomeslarge and the distance between baffle plate 1 and valve body 2 c in thedirection of the flow of exhaust gas becomes large. End peripheralsurface 2 d of valve 2 is apart from contact surface 8 b of end valveguide 8, and there is generated a clearance between end peripheralsurface 2 d and contact surface 8 b. Therefore, the upstream sideexpansion chamber on the upstream side of baffle plate 1 can becommunicated again with the downstream side expansion chamber on thedownstream side of baffle plate 1 through the clearance between endperipheral surface 2 d and contact surface 8 b. Thus, when valve 2 isfurther rotationally moved from position C in the opening direction,valve 2 is in a second open state in which the fluid communicationbetween the upstream side expansion chamber and the downstream sideexpansion chamber which are disposed on the upstream and downstreamsides of baffle plate 1 can be re-established through the clearancebetween end peripheral surface 2 d and contact surface 8 b. Theclearance is increased as the distance between baffle plate 1 and valvebody 2 c in the direction of the flow of exhaust gas becomes larger.Further, in the second open state of valve 2, the opening of valve 2 andthe angular position of valve 2 with respect to baffle plate 1, variesin accordance with the rotation amount of valve 2 and the distancebetween baffle plate 1 and valve body 2 c in the direction of the flowof exhaust gas. That is, as the rotation amount of valve 2 and thedistance between baffle plate 1 and valve body 2 c in the direction ofthe flow of exhaust gas increases, the opening of valve 2 becomeslarger.

Opening 10 and the clearance between baffle plate 1, valve body 2 c andvalve guides 6, 7 and 8 constitute the communication passage between theupstream side expansion chamber disposed on the upstream side of baffleplate 1 and the downstream side expansion chamber disposed on thedownstream side of baffle plate 1. That is, the communication passage isformed by the aperture-forming rim of baffle plate 1, end peripheralsurface 2 d of valve body 2 c, opposed surfaces of side valve guides 6and 7 opposed in the axial direction of valve shaft 4, and clearanceforming surface 8 a and contact surface 8 b of end valve guide 8.

Referring to FIG. 4, there is shown muffler 20 to which the exhaustsound controlling apparatus of the embodiment is applied. FIG. 4 shows asectional view of muffler 20, taken in a direction of the flow ofexhaust gas that passes through muffler 20. Muffler 20 may have agenerally elliptic-shape in cross-section taken in a directionperpendicular to the direction of the flow of exhaust gas. As shown inFIG. 4, baffle plate 21 is disposed within muffler 20 so as to divide aninterior of muffler 20 into two expansion chambers adjacent to eachother, namely, first expansion chamber 22 and second expansion chamber23 which are located on the left side and the right side of FIG. 4,respectively. First expansion chamber 22 and second expansion chamber 23are thus separated from each other by baffle plate 21.

Inlet tube 24 extends into second expansion chamber 23 through anupstream end wall of muffler 20, first expansion chamber 22 and baffleplate 21. Inlet tube 24 has one end that outwardly projects from theupstream end wall of muffler 20 and is connected to the engine throughan exhaust pipe, not shown. Inlet tube 24 has the other end that isopened within second expansion chamber 23. Inlet tube 24 serves as afirst communication tube that communicates second expansion chamber 23with the engine. Inlet tube 24 introduces the exhaust gas from theengine into second expansion chamber 23. Pass-through tube 25 extendsthrough baffle plate 21 and communicates first expansion chamber 22 andsecond expansion chamber 23 with each other. Pass-through tube 25 hasone end that is opened within second expansion chamber 23 and the otherend that is opened within first expansion chamber 22. Tail tube 26extends from first expansion chamber 22 to an outside of muffler 20through baffle plate 21, second expansion chamber 23 and a downstreamend wall of muffler 20. Tail tube 26 has one end that is opened withinfirst expansion chamber 22 and the other end that outwardly projectsfrom the downstream end wall of muffler 20 and is exposed to atmosphericair. Tail tube 26 serves as a second communication tube thatcommunicates first expansion chamber 22 with atmospheric air. Firstexpansion chamber 22 and second expansion chamber 23 are larger incross-sectional area than inlet tube 24, pass-through tube 25 and tailtube 26.

Baffle plate 21 has generally rectangular-shaped aperture 21 a that isdisposed parallel with pass-through tube 25. The valve structuredescribed above with reference to FIG. 1 to FIG. 3 is provided on baffleplate 21 on a side of first expansion chamber 22. That is, valve 2 isarranged on a downstream side surface of baffle plate 21 which isexposed to first expansion chamber 22, such that valve body 2 c coversaperture 21 a except for opening 10. Valve 2, valve guides 6, 7 and 8and baffle plate 21 cooperate with each other to define a communicationpassage that allows fluid communication between first expansion chamber22 and second expansion chamber 23 in parallel with pass-through tube25. The communication passage includes opening 10 that is disposedbetween end peripheral surface 2 d of valve 2 and the aperture-formingrim of baffle plate 21 which defines the periphery of aperture 21 a. Thecommunication passage further includes the clearance that is formedbetween valve 2 and valve guides 6, 7 and 8 upon rotation of valve 2about valve shaft 4. In the construction of muffler 20, the exhaust pathextends through inlet tube 24, pass-through tube 25, tail tube 26, firstexpansion chamber 22, second expansion chamber 23 and the communicationpassage between first expansion chamber 22 and second expansion chamber23.

In thus constructed muffler 20, exhaust sound generated from the engineis controlled in accordance with an operating condition of the engine.The exhaust sound is emitted to atmospheric air via a first route, asecond route or both of the first and second routes in accordance withthe engine operating condition. The first route is indicated by arrow Ain FIG. 4 and the second route is indicated by arrow B in FIG. 4. Thefirst route extends sequentially through inlet tube 24, second expansionchamber 23, pass-through tube 25, first expansion chamber 22 and tailtube 26. The second route extends sequentially through inlet tube 24,the communication passage between valve 2 and valve guides 6, 7 and 8,first expansion chamber 22 and tail tube 26. The second route thusbypasses pass-through tube 25.

Specifically, in an engine starting region in which a flow of exhaustgas from the engine starts at zero and increases, the exhaust sound fromthe engine is emitted from tail tube 26 to atmospheric air via the firstroute indicated by arrow A in FIG. 4. Further, in the engine startingregion, valve 2 is rotationally moved from position A to the positionimmediately before position B as shown in FIG. 3. At this time, valve 2is placed in the first open state in which second expansion chamber 23is communicated with first expansion chamber 22 through thecommunication passage therebetween, namely, through opening 10 and theclearance between end peripheral surface 2 d of valve 2 and clearanceforming surface 8 a of end valve guide 8. In this condition, the exhaustsound from the engine is emitted from tail tube 26 to atmospheric airvia the second route in addition to the first route. Since the emissionof the exhaust sound is performed via both the first route and thesecond route, the exhaust sound that is generated in the engine startingregion can be enhanced as compared to the conventional exhaust soundcontrolling apparatus.

Further, when the engine speed is raised up to a predetermined enginespeed which is in the idling region, valve 2 is rotationally moved toposition B shown in FIG. 3 due to an increase in an exhaust pressureimmediately forward of valve 2, namely, an exhaust pressure withinsecond expansion chamber 23, which is hereinafter referred to as avalve-forward pressure. Valve 2 is placed in the closing state in whichend peripheral surface 2 d is substantially contacted with contactsurface 8 b of end valve guide 8. In the closing state of valve 2, theopening of valve 2 and the sectional area of the communication passageare approximately zero. The exhaust sound from the engine is emitted toatmospheric air substantially only via the first route. As a result, theexhaust sound can be reduced.

When the engine speed is raised up to the low engine speed region inwhich the engine speed is higher than in the idling region, namely, whenthe engine is operated in a slow acceleration state, and the engine ismaintained in the low engine speed region, the valve-forward pressure isfurther increased with the engine speed raise. Due to the increase inthe valve-forward pressure, valve 2 is rotationally moved from positionB to position C shown in FIG. 3. During the rotational movement fromposition B to position C, valve 2 is kept in the closing state.Therefore, the emission of the exhaust sound substantially only via thefirst route is maintained so that the exhaust sound can be reduced.

When the engine speed is further raised up to a high engine speed regionin which the engine speed is higher than in the low engine speed region,namely, when the engine is operated in a rapid acceleration state, thevalve-forward pressure is further increased due to an increase in theflow of exhaust gas from the engine. Valve 2 is further rotationallymoved away from position C and placed in the second open state. In thesecond open state, end peripheral surface 2 d of valve 2 becomes out ofcontact with contact surface 8 b of end valve guide 8 so that there isgenerated a clearance therebetween. Therefore, the exhaust sound fromthe engine is emitted from tail tube 26 to atmospheric air via both thefirst route and the second route. Thus, even when the flow of exhaustgas is increased, increase in pressure loss can be suppressed to therebyenhance the engine output in the high engine speed region.

Referring FIG. 5, there is shown a relationship between the engine speedand the opening of valve 2. As shown in FIG. 5, when the engine speed isin the engine starting region, valve 2 is placed in the first open statedescribed above. Further, when the engine speed is increased to be inthe idling region or the low engine speed region, valve 2 is broughtinto the closing state. When the engine speed reaches the high enginespeed region, valve 2 is brought into the second open state as describedabove.

That is, in the exhaust sound controlling apparatus of the presentinvention, valve 2 is constructed to be operated corresponding to theengine speed. The spring force (spring constant) of torsion spring 5,the opening of valve 2 and the opening area of aperture 1 a, 21 a may besuitably set to operate valve 2 corresponding to the engine speed asdescribed above. Alternatively, the valve-forward pressure (the exhaustpressure in second expansion chamber 23) may be suitably set to operatevalve 2 corresponding to the engine speed.

FIG. 6 shows a relationship between the valve-forward pressure and theflow rate of exhaust gas. As shown in FIG. 6, the valve-forward pressurevaries depending upon the flow rate of exhaust gas. Further, as shown inFIG. 6, as the engine speed becomes higher, the flow rate of exhaust gasis increased in the order of the idling region, the low engine speedregion and the high engine speed region.

On the basis of the relationships as explained above, the constructionof the exhaust sound controlling apparatus of the present invention isappropriately designed. For instance, the spring constant (the biasingforce) of torsion spring 5 can be set such that valve 2 is rotationallymoveable from position A to the position immediately before position Bagainst the valve-forward pressure in the engine starting region, valve2 is rotationally moveable from position B to position C against thevalve-forward pressure in the idling region and the low engine speedregion, and valve 2 is rotationally moveable beyond and away fromposition C in the high engine speed region, as shown in FIG. 3.

Owing to suitably setting the spring constant of torsion spring 5 asdescribed above, the rotational movement of valve 2 from position B andposition C can be ensured even when the flow of exhaust gas pulses inthe idling region and the low engine speed region and the rotationalmovement of valve 2 pulses along with the pulse of the flow of exhaustgas. Valve 2, therefore, can be prevented from coming into contact withbaffle plate 1, 21. As a result, when the engine is operated in theidling region and the low engine speed region, it is possible tosuppress occurrence of striking noise which would be caused due to thecontact of valve 2 with baffle plate 1, 21, and maintain a silencewithin the interior of the vehicle.

The exhaust sound controlling apparatus of the embodiment can performthe following function and effect. In the engine starting region, valve2 is placed in the first open state in which exhaust gas emitted fromthe engine is permitted to pass through the communication passage in thesecond route B and thereby flow in the exhaust path via both the firstroute A and the second route B as shown in FIG. 4. As a result, powerfulexhaust sound can be created. In the idling region and the low enginespeed region, valve 2 is placed in the closing state in which the flowof exhaust gas is prevented from passing through the communicationpassage in the second route B and thereby allowed to flow in the exhaustpath substantially only via the first route A. As a result, a silencewithin the interior of the vehicle can be ensured. In the high enginespeed region, valve 2 is placed in the second open state in which theflow of exhaust gas is permitted to pass through the communicationpassage in the second route B and thereby flow in the exhaust path viaboth the first route A and the second route B, and the opening of valve2 is increased as the engine speed becomes higher. Therefore, in thehigh engine speed region, even when an amount of the flow of exhaust gasis increased, increase in pressure loss can be suppressed.

Further, with the provision of the simple valve structure as describedabove, the exhaust sound control can be suitably performed depending onthe engine speed as explained above. Further, the exhaust sound can becontrolled by setting the spring constant (the biasing force) of torsionspring 5 as explained above.

Furthermore, curved end portion 2 a of valve 2 serves as a gas leakageinhibiting portion that prevents the exhaust gas and the exhaust soundfrom leaking from curved end portion 2 a toward the downstream side ofbaffle plate 1 during the rotational movement of valve 2. Valve 2 issupported at curved end portion 2 a relative to baffle plate 1, and theouter circumferential surface of curved end portion 2 a is substantiallyin contact (slide contact) with baffle plate 1 during the rotationalmovement of valve 2. Therefore, while valve 2 is rotationally moved, theexhaust gas and the exhaust sound can be prevented from leaking fromcurved end portion 2 a toward the downstream side of baffle plate 1.

Referring to FIG. 7 to FIG. 9, a modification of the valve structureused in the exhaust sound controlling apparatus of the embodiment isexplained. Similar to the valve structure as described above, themodified valve structure is constructed to prevent the exhaust gas fromleaking from the support portion of valve 102 with respect to baffleplate 1. As shown in FIG. 7 and FIG. 8, valve 102 is a generallyrectangular-shaped plate member and differs from valve 2 used in theembodiment in that a curved end portion is not formed. Valve 102includes planar valve body 2 c with one end portion 2 e, and a pair ofsupport portions 2 b disposed on opposite sides of one end portion 2 e.Valve shaft 4 extends through shaft insertion holes of support portions2 b and is supported by support portions 2 b. Each of support portions 2b has an extension that extends around the shaft insertion hole, and isconnected at the extension with one end portion 2 e.

Gas leakage inhibiting member 11 is disposed on the downstream sidesurface of baffle plate 1 and located on a side of one end portion 2 eof valve body 2 c. Gas leakage inhibiting member 11 extends parallel tovalve shaft 4 or one end portion 2 e of valve body 2 c, namely, an upperend periphery of valve 102 when viewed in FIG. 7 and FIG. 8. Gas leakageinhibiting member 11 is a bar member that has a generally L-shape incross-section as shown in FIG. 8. Gas leakage inhibiting member 11 hasone strip portion 11 a that is mounted to baffle plate 1 and the otherstrip portion 11 b that is disposed uprightly with respect to baffleplate 1. Specifically, the other strip portion 11 b is inclined towardaperture 1 a of baffle plate 1.

As shown in FIG. 9, when valve 102 is rotationally moved about valveshaft 4, a tip end of one end portion 2 e of valve 102 is moved so as todraw a part of a circle that has a center on the axis of valve shaft 4.At this time, the tip end of one end portion 2 e is moved on an outersurface of the other strip portion 11 b of gas leakage inhibiting member11 in contact with the outer surface of the other strip portion 11 b.

During the rotational movement of valve 102, the exhaust gas and theexhaust sound can be prevented from leaking from one end portion 2 e ofvalve 102 at which valve 102 is supported relative to baffle plate 1through gas leakage inhibiting member 11, toward the downstream side ofbaffle plate 1. In view of this effect of gas leakage prevention, it ispreferred that the other strip portion 11 b of gas leakage inhibitingmember 11 is curved in conformity with a locus of the tip end of one endportion 2 e of valve 102 which is drawn during the rotational movementof valve 102.

Valve 102 has the same operating positions and operating states as thoseof valve 2 as shown in FIG. 3. That is, in the engine starting region,valve 102 is moveable from position A to a position immediately beforeposition B as shown in FIG. 9. At this time, valve 102 is in the firstopen state in which the fluid communication between the upstream sideexpansion chamber on the upstream side of baffle plate 1 and thedownstream side expansion chamber on the downstream side of baffle plate1 through the communication passage therebetween is established. In theidling region and the low engine speed region, valve 102 is moved fromposition B to position C as shown in FIG. 9. At this time, valve 102 isin the closing state in which the fluid communication between theupstream side expansion chamber on the upstream side of baffle plate 1and the downstream side expansion chamber on the downstream side ofbaffle plate 1 through the communication passage therebetween isblocked. In the high engine speed region, valve 102 is moved away fromposition C in the opening direction. At this time, valve 102 is in thesecond open state in which the fluid communication between the upstreamside expansion chamber on the upstream side of baffle plate 1 and thedownstream side expansion chamber on the downstream side of baffle plate1 through the communication passage therebetween is re-established.

Referring to FIG. 10, a modification of muffler 20 shown in FIG. 4 isexplained. As shown in FIG. 10, modified muffler 40 includes firstbaffle plate 41 and second baffle plate 42 that extend within muffler 40in a cross-sectional direction of muffler 40, namely, in a directionperpendicular to the flow of exhaust gas that passes through muffler 40.Baffle plates 41 and 42 are disposed within muffler 40 so as to dividethe interior of muffler 40 into three expansion chambers adjacent toeach other, namely, first expansion chamber 43, second expansion chamber44 and third expansion chamber 45, which are located from the left sideto the right side of muffler 40 in this order. First expansion chamber43 and second expansion chamber 44 are separated from each other bybaffle plate 41. Second expansion chamber 44 and third expansion chamber45 are separated from each other by baffle plate 42. Second expansionchamber 44 is disposed between first expansion chamber 43 and thirdexpansion chamber 45.

Inlet tube 46 extends into second expansion chamber 44 through anupstream end wall of muffler 40, first expansion chamber 43 and firstbaffle plate 41. Inlet tube 46 has one end that outwardly projects fromthe upstream end wall of muffler 40 and is connected to the enginethrough an exhaust pipe, not shown. Inlet tube 46 has the other end thatis opened within second expansion chamber 44. Inlet tube 46 introducesthe exhaust gas from the engine into second expansion chamber 44.

First pass-through tube 47 is disposed within muffler 40 coaxially withinlet tube 46. First pass-through tube 47 extends from second expansionchamber 44 into third expansion chamber 45 through second baffle plate42 and communicates second expansion chamber 44 and third expansionchamber 45 with each other. First pass-through tube 47 has one end thatis opened within second expansion chamber 44 and the other end that isopened within third expansion chamber 45 with each other. Secondpass-through tube 48 extends through second baffle plate 42 in parallelwith first pass-through tube 47. Second pass-through tube 48 has one endthat is opened within third expansion chamber 45 and the other end thatis connected with one side surface of first baffle plate 41 which isexposed to second expansion chamber 44.

First baffle plate 41 has generally rectangular-shaped aperture 41 a.Second pass-through tube 48 is disposed coaxially with aperture 41 a soas to communicate third expansion chamber 45 with aperture 41 a. Thirdexpansion chamber 45 and first expansion chamber 43 are communicatedwith each other through aperture 41 a and second pass-through tube 48.The valve structure shown in FIG. 1 to FIG. 3 or the modified valvestructure shown in FIG. 7 to FIG. 9 is provided on an opposite sidesurface of first baffle plate 41 which is exposed to first expansionchamber 43. That is, valve 2 or 102 is arranged on the opposite sidesurface of first baffle plate 41, namely, on a downstream side surfaceof first baffle plate 41 with respect to the flow of exhaust gas passingthrough second pass-through tube 48, such that valve body 2 c coversaperture 41 a except for opening 10. Valve 2 or 102, valve guides 6, 7and 8 and first baffle plate 41 cooperate with each other to define acommunication passage that allows fluid communication between firstexpansion chamber 43 and third expansion chamber 45 through secondpass-through tube 48. The communication passage includes opening 10 thatis disposed between end peripheral surface 2 d of valve 2 or 102 and anaperture-forming rim of first baffle plate 41 which defines theperiphery of aperture 41 a. The communication passage further includesthe clearance that is formed between valve 2 or 102 and valve guides 6,7 and 8 upon rotation of valve 2 or 102 about valve shaft 4.

First tail tube 49 extends from first expansion chamber 43 to an outsideof muffler 40 through first baffle plate 41, second baffle plate 42,second expansion chamber 44, third expansion chamber 45 and a downstreamend wall of muffler 40. First tail tube 49 has one end that is openedwithin first expansion chamber 43 and the other end that outwardlyprojects from the downstream end wall of muffler 40 and is exposed toatmospheric air. Second tail tube 50 extends from second expansionchamber 44 to the outside of muffler 40 through second baffle plate 42,third expansion chamber 45 and the downstream end wall of muffler 40.Second tail tube 50 has one end that is opened within second expansionchamber 44 and the other end that outwardly projects from the downstreamend wall of muffler 40 and is exposed to atmospheric air. Each ofexpansion chambers 43, 44, 45 is larger in cross-sectional area thaninlet tube 46, pass-through tubes 47, 48 and tail tubes 49, 50. In theconstruction of muffler 40, the exhaust path extends through first inlettube 46, second expansion chamber 44, second tail tube 50, firstpass-through tube 47, third expansion chamber 45, second pass-throughtube 48, first expansion chamber 43, first tail tube 49 and thecommunication passage between first expansion chamber 43 and thirdexpansion chamber 45.

In thus constructed muffler 40, exhaust sound generated from the engineis controlled depending on an operating condition of the engine. Theexhaust sound is emitted to atmospheric air via a first route indicatedby arrow A in FIG. 10, a second route indicated by arrow B in FIG. 10 orboth the first route and the second route in accordance with the engineoperating condition. The first route extends sequentially through inlettube 46, second expansion chamber 44 and second tail tube 50. The secondroute extends sequentially through inlet tube 46, second expansionchamber 44, first pass-through tube 47, third expansion chamber 45,second pass-through tube 48, the communication passage between valve 2or 102 and valve guides 6, 7 and 8, first expansion chamber 43 and firsttail tube 49. The second route bypasses second tail tube 50.

Specifically, in the engine starting region, the exhaust sound from theengine is emitted from second tail tube 50 to atmospheric air via thefirst route indicated by arrow A in FIG. 10. Further, in the enginestarting region, valve 2 or 102 is moveable from position A to aposition immediately before position B as shown in FIG. 9. At this time,valve 2 or 102 is placed in the first open state in which thecommunication passage including opening 10 and the clearance between endperipheral surface 2 d of valve 2 or 102 and clearance forming surface 8a of end valve guide 8 is opened so that the second route indicated byarrow B in FIG. 10 is permitted. Therefore, exhaust sound from theengine is also emitted from first tail tube 49 to atmospheric air viathe second route. Since the emission of the exhaust sound is thusperformed via both the first route and the second route, the exhaustsound in the engine starting region can be enhanced as compared to theconventional exhaust sound controlling apparatus.

Further, when the engine speed is raised up to the idling region, valve2 or 102 is rotationally moved to position B shown in FIG. 9 due to anincrease in the valve-forward pressure. Valve 2 or 102 is placed in theclosing state in which end peripheral surface 2 d is substantiallycontacted with contact surface 8 b of end valve guide 8. Therefore, theexhaust sound from the engine is prevented from being emitted from tailtube 49 to atmospheric air via the second route indicated by arrow B inFIG. 10 and can be emitted from tail tube 50 to atmospheric airsubstantially only via the first route indicated by arrow A in FIG. 10.As a result, the exhaust sound that is emitted from muffler 40 toatmospheric air can be reduced.

When the engine speed is further raised up to the low engine speedregion and maintained in the low engine speed region, the valve-forwardpressure is further increased with raising of the engine speed. Due tothe increase in the valve-forward pressure, valve 2 or 102 isrotationally moved from position B to position C shown in FIG. 9. Duringthe rotational movement from position B to position C, valve 2 or 102 iskept in the closing state. Therefore, the emission of the exhaust soundsubstantially only via the first route indicated by arrow A in FIG. 10is maintained so that the exhaust sound from the engine can be keptreduced.

Further, when the engine speed is raised up to the high engine speedregion, the valve-forward pressure is further increased due to anincrease in the flow of exhaust gas from the engine. Valve 2 or 102 isfurther rotationally moved away from position C shown in FIG. 9 andplaced in the second open state. In the second open state, endperipheral surface 2 d of valve 2 or 102 becomes out of contact withcontact surface 8 b of end valve guide 8 and there is generated aclearance therebetween. Therefore, the exhaust sound from the engine isemitted from both tail tubes 49 and 50 to atmospheric air via both thefirst route indicated by arrow A in FIG. 10 and the second routeindicated by arrow B in FIG. 10. Thus, in the high engine speed region,even when the flow of exhaust gas is increased, increase in pressureloss can be suppressed to thereby enhance the engine output.

Next, a method for controlling exhaust sound from the engine which isemitted through the exhaust path, of the embodiment is explained. Themethod includes a first control step of reducing a flow rate of exhaustgas that passes through the exhaust path, to a first flow rate inaccordance with increase in engine speed in a first engine speed region,and a second control step of increasing the flow rate of exhaust gas toa second flow rate in accordance with increase in engine speed in asecond engine speed region in which the engine speed is higher than theengine speed in the first engine speed region. The method furtherincludes a third control step of reducing the flow rate of exhaust gasto a third flow rate that is lower than the first flow rate and thesecond flow rate in a third engine speed region between the first enginespeed region and the second engine speed region. The engine speed in thefirst engine speed region includes an engine speed in an engine startingregion. The engine speed in the second engine speed region includes anengine speed in a high engine speed region. The engine speed in thethird engine speed region includes an engine speed in an idling regionand an engine speed in a low engine speed region.

The method for controlling exhaust sound can perform the followingeffects. In the engine starting region, exhaust gas emitted from theengine is permitted to pass through the exhaust path so that powerfulexhaust sound can be created. In the idling region and the low enginespeed region, the flow of exhaust gas is reduced so that a silencewithin the interior of the vehicle can be ensured. In the high enginespeed region, the flow of exhaust gas is permitted to pass through theexhaust path and the flow rate of exhaust gas is increased as the enginespeed becomes higher. As a result, increase in pressure loss can besuppressed.

This application is based on prior Japanese Patent Application No.2007-098526 filed on Apr. 4, 2007. The entire contents of the JapanesePatent Application No. 2007-098526 are hereby incorporated by reference.

Although the invention has been described above by reference to acertain embodiment of the invention and modifications of the embodiment,the invention is not limited to the embodiment and the modificationsdescribed above. Variations of the embodiment and the modificationsdescribed above will occur to those skilled in the art in light of theabove teachings. The scope of the invention is defined with reference tothe following claims.

1. A method for controlling exhaust sound from an engine which isemitted through an exhaust path, the method comprising: a first controlstep of reducing a flow rate of exhaust gas that passes through theexhaust path, to a first flow rate in accordance with increase in enginespeed in a first engine speed region; and a second control step ofincreasing the flow rate of exhaust gas to a second flow rate inaccordance with increase in engine speed in a second engine speed regionin which the engine speed is higher than that in the first engine speedregion.
 2. The method as claimed in claim 1, further comprising a thirdcontrol step of reducing the flow rate of exhaust gas to a third flowrate that is lower than the first flow rate and the second flow rate ina third engine speed region between the first engine speed region andthe second engine speed region.
 3. The method as claimed in claim 1,wherein the engine speed in the first engine speed region comprises anengine speed in an engine starting region.
 4. The method as claimed inclaim 1, wherein the engine speed in the second engine speed regioncomprises an engine speed in a high engine speed region.
 5. The methodas claimed in claim 2, wherein the engine speed in the third enginespeed region comprises an engine speed in an idling region.
 6. Themethod as claimed in claim 2, wherein the engine speed in the thirdengine speed region comprises an engine speed in a low engine speedregion.
 7. An apparatus for controlling exhaust sound from an enginewhich is emitted through an exhaust path, the apparatus comprising: abaffle plate that is disposed within the exhaust path so as to block aflow of exhaust gas in the exhaust path, the baffle plate being formedwith an aperture; a communication passage that communicates an upstreamside of the baffle plate and a downstream side of the baffle plate witheach other; a valve that is so disposed as to cover a part of theaperture of the baffle plate and operated to open and close thecommunication passage; and wherein the valve is operated to open thecommunication passage in an engine starting region, the valve isoperated to close the communication passage in an idling region and alow engine speed region, and the valve is operated to open thecommunication passage and increase an opening of the valve as the enginespeed rises in a high engine speed region.
 8. The apparatus as claimedin claim 7, further comprising a means for cooperating with the valve todefine the communication passage during the rotational movement of thevalve.
 9. The apparatus as claimed in claim 7, further comprising abiasing means for biasing the valve toward the baffle plate.
 10. Anapparatus for controlling exhaust sound from an engine which is emittedthrough an exhaust path, the apparatus comprising: a baffle plate thatis disposed within the exhaust path so as to block a flow of exhaust gasin the exhaust path, the baffle plate being formed with an aperture; avalve that is so disposed as to cover a part of the aperture of thebaffle plate, the valve being rotationally moveable relative to thebaffle plate so as to be apart from the baffle plate in a direction ofthe flow of exhaust gas, a biasing means for biasing the valve towardthe baffle plate; and an enclosure member that extends from the baffleplate so as to be opposed to the outer periphery of the valve when thevalve is moved relative to the baffle plate, wherein the valve has acontact position in which the valve is contacted with the baffle plateand covers the part of the aperture of the baffle plate, a smalldistance position in which the valve is apart from the baffle plate inthe direction of the flow of exhaust gas with a first distancetherebetween, an intermediate distance position in which the valve isapart from the baffle plate in the direction of the flow of exhaust gaswith a second distance therebetween which is larger than the firstdistance, and a large distance position in which the valve is apart fromthe baffle plate in the direction of the flow of exhaust gas with athird distance therebetween which is larger than the first distance andthe second distance, and wherein when the valve is placed in the contactposition, the valve cooperates with the baffle plate to define anopening between the outer periphery of the valve and a periphery of theaperture of the baffle plate, when the valve is placed in the smalldistance position, the valve cooperates with the enclosure member todefine a first clearance between the outer periphery of the valve and anopposing surface of the enclosure member that is opposed to the outerperiphery of the valve, when the valve is placed in the intermediatedistance position, at least a part of the outer periphery of the valveis substantially contacted with the opposing surface of the enclosuremember, and when the valve is placed in the large distance position, thevalve cooperates with the enclosure member to define a second clearancebetween the outer periphery of the valve and the opposing surface of theenclosure member, and the second clearance is increased as the thirddistance between the valve and the baffle plate becomes larger.
 11. Theapparatus as claimed in claim 10, wherein the biasing means has abiasing force that acts on the valve against a pressure of the exhaustgas that flows in the exhaust path, the biasing force urges the valve tomove to the small distance position in an engine starting region, thebiasing force urges the valve to move to the intermediate distanceposition in an idling region and a low engine speed region, and thebiasing force urges the valve to move to the large distance position ina high engine speed region.
 12. The apparatus as claimed in claim 10,wherein the valve comprises a gas leakage inhibiting portion thatprevents the exhaust gas and the exhaust sound from leaking from asupport portion of the valve at which the valve is supported relative tothe baffle plate, toward a downstream side of the baffle plate duringthe rotational movement of the valve.
 13. The apparatus as claimed inclaim 10, further comprising a gas leakage inhibiting member thatprevents the exhaust gas and the exhaust sound from leaking from asupport portion of the valve at which the valve is supported relative tothe baffle plate, toward a downstream side of the baffle plate duringthe rotational movement of the valve.
 14. The apparatus as claimed inclaim 10, further comprising a first expansion chamber, a secondexpansion chamber that are separated from the first expansion chamber bythe baffle plate, a first communication tube having one end that isconnected to the engine and the other end that is opened within thesecond expansion chamber, a second communication tube having one endthat is exposed to atmospheric air and the other end that is openedwithin the first expansion chamber, and a third communication tube thatextends through the baffle plate and communicates the first expansionchamber and the second expansion chamber with each other, wherein thefirst and second expansion chambers are larger in sectional area thanthe first, second and third communication tubes.
 15. The apparatus asclaimed in claim 10, further comprising a first expansion chamber, asecond expansion chamber that is separated from the first expansionchamber by the baffle plate, a third expansion chamber, a second baffleplate that separates the third expansion chamber and the secondexpansion chamber from each other, an inlet tube having one end that isconnected to the engine and the other end that is opened within thesecond expansion chamber, a first tail tube having one end that isexposed to atmospheric air and the other end that is opened within thefirst expansion chamber, a second tail tube having one end that isexposed to atmospheric air and the other end that is opened within thesecond expansion chamber, a first pass-through tube that extends throughthe second baffle plate and communicates the second expansion chamberwith the third expansion chamber, and a second pass-through tube thatextends through the second baffle plate and communicates the thirdexpansion chamber with the aperture of the baffle plate, wherein thefirst, second and third expansion chambers being larger in sectionalarea than the inlet tube, the first and second tail tubes and the firstand second pass-through tubes.